This invention relates generally to the suppression of transient voltages, and more particularly to an improved thermal fusing arrangement for protecting transient voltage surge protectors of the varistor type, most particularly the metal oxide varistor type.
Varistors, more particularly metal oxide varistors, have been used to protect electrical circuits from the deleterious effects of transient voltage spikes for many years. Metal oxide varistors, more particularly zinc oxide varistors, have the capability of clamping high transient voltages appearing on unconditioned power lines to a low level for protecting electrical equipment or devices connected to the line.
In the simplest arrangement, a metal oxide varistor can simply be connected between the hot and neutral conductors of a single phase electrical circuit, in parallel between the line and the load. Transient voltages exceeding the threshold voltage of the varistor are shunted by the varistor.
While metal oxide varistors have a long life, and have the ability to repeatedly clamp high transient voltage spikes to a safe level, they do eventually fail and ultimately, even if a catastrophic failure does not occur, the impedance of metal oxide varistors decreases to the point where they present a significant load, and eventually overheat and fail emitting smoke and fumes.
It is known to provide protection for metal oxide varistors by connecting the varistors across the power line in series with a current limiting fuse and/or a thermal fuse. If the impedance of the varistor decreases to the point where a current in excess of the designed current for the current limiting fuse is exceeded, the fuse will open and the varistor will be effectively removed from the circuit. Similarly, even if the current does not rise to a level sufficient to open the current limiting fuse, if the temperature of the varistor increases beyond the rated temperature of the thermal-fuse, the thermal fuse will open, thereby removing the varistor from the circuit.
The thermal-fuses heretofore used to protect electrical circuits from varistor failure are generally cylindrical in shape, and have been mounted on the same printed circuit boards to which the varistor is mounted with the cylindrical body of the fuse arranged adjacent and parallel to the varistor body. As long as the thermal protective fuse is physically close enough to the varistor, an increase in varistor temperature will increase the temperature of the thermal protective fuse, causing it to open. Even though thermal protective fuses of the type heretofore used to protect electrical circuits from varistor failure have been somewhat effective, varistors may overheat in failure at a physical location on the varistor body, that is remote from the thermal protective fuse. FIG. 1 shows a printed circuit board to which a metal oxide varistor and a cylindrical thermal protective fuse are mounted. As long as the varistor body overheats sufficiently at a location close to the fuse, the fuse will be effective. However, if localized overheating occurs at a portion of the varistor body remote from the fuse, the varistor and surrounding areas may be substantially destroyed before the temperature at the location of the fuse increases sufficiently to cause the fuse to open.
Accordingly, it is an object of this invention to provide a thermal protective arrangement for metal oxide varistors that overcomes the problems just mentioned. Briefly stated, and in accordance with a presently preferred aspect of the invention, thermal protective fuse having a cylindrical body with a thermally protective portion disposed substantially adjacent to one end of the cylindrical body is mounted in close proximity with the body of a metal oxide varistor, with the thermally responsive portion of the fuse positioned approximately adjacent the center of the varistor body.
In accordance with another aspect of this invention, the cylindrical body of the thermally responsive fuse is arranged substantially vertically with the thermally responsive portion on top relative to the printed circuit board, generally adjacent the center of the varistor body.